Fiber optics are widely used in many diverse applications, including telecommunication systems, instrumentation and sensing operations. An example of such an application is a multi-access optical telecommunications network. In such a network, optical fiber connects a number of users or subscribers to a central office using passive couplers. This type of network is particularly attractive since there are typically no active optical devices located outside of the central office or subscriber locations.
An optical fiber typically includes an inner glass or plastic core surrounded by an outer cladding similarly of glass or plastic. The inner core has a relatively higher index of refraction than the cladding thus allowing light to be transmitted through the core very efficiently. Light may be transferred or split between separate fibers through the use of an optical fiber coupler. One extensively used type of optical fiber coupler is a fused biconically tapered (FBT) coupler. In one method of producing such a fiber optic coupler, a number of optical fibers are held in axial alignment and elongated while being heated. This process creates a biconically tapered region or waist wherein the optical fibers are fused together and the optical signals from one or more optical fibers can be coupled to or split between other optical fibers.
The basic optical performance of optical fiber couplers can be described by three fundamental quantities: excess loss, splitting loss and uniformity. The excess loss, expressed in decibels (dB), is a measure of how much light or optical energy is lost in the coupling process. Excess loss is defined as the ratio of the total output power to the amount of optical power launched into the input of the coupler. The ratio of the optical power in one of the output fibers relative to the total optical power output over all the output fibers is known as the splitting loss. The splitting loss is also often expressed in decibels. Another term often used to characterize the actual optical performance of couplers is "uniformity". Uniformity is a measure of the spread in the splitting ratios from the ideal values. It is also expressed in decibels and is defined as the difference between the maximum and minimum values of splitting loss.
Many of the optical fiber couplers in use today are designed to operate effectively over only a narrow range or "window" of wavelengths. The most common wavelengths of interest for telecommunication applications are those centered around 1300 nm or 1550 nm. These optical fiber couplers, often called single window couplers, essentially provide equal splitting of light from one or more input fibers to a number of output fibers at a preselected wavelength.
However, the splitting loss of each output port for such a single window coupler changes as a function of the wavelength of the transmitted light. In particular, as the wavelength of the transmitted light varies from the center of the wavelength window, the optical power in the output fibers (i.e., splitting loss) tend to diverge from the ideal value and the uniformity becomes quite large. This behavior typically limits the use of such single window couplers to within .+-.20 nm of the center of the wavelength window.
In many optical fiber telecommunication applications, operation in two wavelength windows, such as 1300 nm and 1550 nm, is required in order to provide both telephony and broadband services. In these applications broadband optical fiber couplers, which exhibit a relatively constant splitting loss over a broad range of wavelengths, are required.
One technique of fabricating a broadband optical fiber coupler requires introducing a dissimilarity in one of two optical fibers in the coupling region. This technique, however, has been limited to producing couplers with only two output fibers. Other techniques which allow splitting over more than two output fibers are known, but they are limited to 1.times.N couplers. One such technique involves inserting identical stripped optical fibers into a tight fitting outer sheath consisting of a glass capillary tube. The entire structure is then heated and stretched to achieve coupling. By varying the number of fibers inserted into the glass sheath and their relative positions and separations, certain broadband 1.times.N couplers can be fabricated. However, most of these 1.times.N couplers require either the use of N+1 fibers or the use of dummy fibers that have no light guiding cores and as such are not conventional optical fibers.
It would be desirable to provide an M.times.N broadband optical fiber coupler, where M ranges from 1 to N and N is greater than 2, that is easily fabricated and has good uniformity and splitting ratios and low excess losses.